How do certain Affiliate Marketing companies know I'm using proxies?

In the past I have used a vast array of sign up service VPNs, private proxies, etc. However on some new Affiliate Marketing companies there seems to be some sort of system in place that ALWAYS know when I'm using a proxy/VPN because instead of taking me to the landing page, it redirects me to some other page instead. And yes these are US proxies/VPNs. I feel the services I'm using are pretty obscure (including some private proxy services from this forum) How could they possibly have a database of every "non real IP" or are they detecting it some other way? Anyone have any advice? This is driving me nuts.

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